Just to expound a little on that answer, when the above poster says “clear its orbit”, they are referring to ability of full-sized planets to clear their surroundings of debris. Pluto is unable to do so because it lacks sufficient gravity. Pluto is therefore not considered gravitationally dominant, instead orbiting the sun in a “zone of similar objects” – in this case, the Kuiper belt. However, there is ongoing debate within the astronomical community as to this classification of planets (it was introduced only in 2006, by the International Astronomical Union). For the time being however, Pluto, along with Ceres and Eris (as well as 50 possible candidates in the Kuiper Belt), are relegated to dwarf status.